THE OROCOPIA-PELONA SCHIST TERRANE, SOUTHERN CALIFORNIA AND SOUTHWEST ARIZONA: A CALIBRATED TARGET FOR CRUSTAL SEISMIC ANISOTROPY STUDIES

The Orocopia-Pelona Schist belt represents an enigmatic element within the Late Cretaceous to early Tertiary tectonic framework of the southern Cordillera. While not conclusively known, regional depth and thickness, breadth, and large-scale internal configuration of the schist differ according to current evolutionary models: (1) low-angle, eastward subduction, (2) trapping of marine rocks along a transpressional continental margin, or (3) tectonic burial of Great Valley-like forearc strata beneath overriding Mesozoic batholithic arc crust. The three-D distribution of the schist could help to constrain these models, but is poorly known. The Orocopia-Pelona Schist is thus a promising candidate for mapping via crustal seismic anisotropy.

The Orocopia-Pelona Schist terrane is exposed in some 15 tectonic windows within SE California and SW Arizona, particularly in tight alignment along the Chocolate Mtns. anticlinorium. This terrane comprises predominantly homogeneous quartzofeldspathic metagraywacke, with subordinate to minor interlayered metabasalt, metachert, siliceous marble, and ultramafic rock. The schist has a regionally strong subhorizontal foliation and a pronounced lineation with dominant NE-SW trend. Metamorphic mineral assemblages indicate tectonic burial to 25 - 35 km depths.

We have made petrophysical lab measurements on samples of Orocopia Schist from the Chocolate and Trigo Mountains, determining seismic velocities and densities at pressures up to 10 kb. Due to the pronounced fabric anisotropy of the schist, compressional and shear wave velocity measurements were made parallel to the three principal axes and also in the directions of the three axial-plane diagonals. The velocity anisotropy exhibits hexagonal to strongly orthorhombic symmetry. We present data for the fabric of the schist, the anisotropic velocities, and the regional distribution and internal character of the schist terrane from the perspective of seismic wave propagation.